Do the consecrated bread and wine cease to be the Body and Blood of Christ when the Mass is over?

No. During the celebration of the Eucharist, the bread and wine become the Body and Blood of Christ, and this they remain. They cannot turn back into bread and wine, for they are no longer bread and wine at all. There is thus no reason for them to change back to their “normal” state after the special circumstances of the Mass are past. Once the substance has really changed, the presence of the Body and Blood of Christ “endures as long as the Eucharistic species subsist” ( Catechism, no. 1377). Against those who maintained that the bread that is consecrated during the Eucharist has no sanctifying power if it is left over until the next day, St. Cyril of Alexandria replied, “Christ is not altered, nor is his holy body changed, but the power of the consecration and his life-giving grace is perpetual in it” ( Letter 83, to Calosyrius, Bishop of Arsinoe [ PG 76, 1076]). The Church teaches that Christ remains present under the appearances of bread and wine as long as the appearances of bread and wine remain (cf. Catechism, no. 1377).